I think plain writing is just what you think it is or to me I would say not joined up and how you see it on computers like the font Ariel.
Hope this helps
what is plain english
In writing, plain style uses simple diction, simple sentence structure, and clear images.
LogoSportswear.com has the plain gray hoodies.
Plain style is writing with simple diction, simple sentence structure, and clear images.
Plain style writing is a clear and straightforward way of communicating information without unnecessary embellishments or complex language. It focuses on clarity and simplicity to ensure that the message is easily understood by the audience. This style is often used in technical writing, journalism, and non-fiction.
The Plain Style was a technique used by the Puritans. This style was characterized by simple, direct, and unambiguous rhetoric, speech, or writing.
Plain style writing... Americanized English
the plain style and typology
It's quite simple; plain and quick.
Started with Puritan plain style, then Romanticism, Transcendentalism, Realism, Then now Modernism
The plain style and typology
use plain paper that measures 81/2 by 11 inches.